On Tuesday President Hosni Mubarak appealed to Egyptians to take part in the referendum. In a short recorded speech on state television, he said the referendum would be "a decisive moment in our contemporary history".
But Aljazeera reported that a group of Egyptian lawyers and trade unionists appealed for a boycott of the referendum, called to decide the fate of an amendment to Article (76) of the Egyptian constitution.
The group on Tuesday slammed the Egyptian government and the ruling party, and described the amendment as a means to bypass the constitution. Members said the amendment did not provide equal opportunity for all presidential candidates.
Egypt's secular and Islamist opposition has called for a boycott of the referendum, arguing that the amendment, overwhelmingly voted by MPs on 10 May, still largely favours candidates of Mubarak's ruling party.
Also on Tuesday, the Egyptian professionals' union held a conference to press their demand for the release of political detainees, cancellation of the emergency law, and for public liberties to be guaranteed, Aljazeera reported, adding that the meeting was attended by the Muslim Brotherhood, the Tagammu party and al-Amal and Kifaya movement representatives.
Opposition leaders have called
for a boycott of the referendum
Political analysts in Egypt have been looking forward to Wednesday's referendum after the Egyptian judiciary overruled the opposition's plea to get it cancelled, Aljazeera said.
Less than half of nine million newly eligible voters - mostly young Egyptians who have reached the voting age of 18 since the last polls - failed to pick up their polling card in time, the Interior Ministry said.
While the ruling party has intensified its campaign to get Egyptians to take part in the referendum, the major opposition parties and the Muslim Brotherhood have reiterated their boycott call, Aljazeera added.
But the ministry said those listed on the electoral roll could vote by simply showing their identification papers - a measure the opposition denounces as liable to encourage fraud if papers are not systematically checked.
More than 54,000 voting booths have been set up across Egypt, with a ratio of one booth for every 600 electors.
Only one polling station in 10 - or 5000 - will be supervised by a judge, in line with the law.
But some of Egypt's 13,000 judges have warned they may not turn up on Wednesday owing to unfulfilled demands for more independence from the executive.
The opposition has been leading a
spirited anti-Mubarak campaign
Ballot boxes are to be counted by 329 electoral commissions, each under judiciary supervision.
The polling card is pink-coloured and the ballot paper simply states: "Are you in agreement with the modification of Article 76 of the constitution by the People's Assembly?"
Voters must tick a "yes" or "no" box, and can be provided with the text of amendment if they so request.
In his televised message on Tuesday, Mubarak said: "I have full and unlimited confidence that you will turn out to take part, through the referendum, in making a new tomorrow for our country and exploring new and broad horizons in our political life."
The Egyptian president is expected to seek a fifth six-year term in contested elections in September.
Police in Cairo earlier on Tuesday dispersed a small demonstration against the constitutional amendment, which sets tough conditions for people who want to run for the presidency. It was by the liberal Ghad (Tomorrow) Party led by young lawyer Ayman Nour.
"I have full and unlimited confidence that you will ... explore new and broad horizons in our political life"
President Hosni Mubarak
Aljazeera's correspondent in Cairo reported earlier on Tuesday that security authorities were keeping up the heat on the Islamist opposition movement.
They detained 15 more members of the banned but tolerated Muslim Brotherhood, for questioning on charges that they possessed leaflets calling for a boycott of the referendum.
The activists were arrested on Tuesday in the Kafr al-Sheikh and Ismailiya governorates, raising the total number of detainees to 900, according to Muslim Brotherhood sources.